1. Field of the Invention
This invention relates generally to the power supply systems that include DC-to-DC conversion operations. More particularly, this invention relates to an improved circuit design and configuration with an asymmetrical full bridge DC-to-DC converter with wide input voltage range.
2. Description of the Prior Art
Conventional art of design and manufacture of a DC-to-DC converter is still limited by lacking of an optimal topology that is suitable for high input voltage and wide input range operations.
FIG. 1 shows the configuration and key operation waveforms of single-ended forward DC-to-DC converter. In this type converter only one switch S1 is employed in the transformer primary side. When the switch S1 turns on, the transformer primary winding is connected to the input voltage Vin, the energy is delivered from source to load by the transformer coupling. When the switch S1 turns off, the transformer primary winding is connected to the magnetic reset circuit MR, which generates a negative voltage xe2x88x92Vr applying to the transformer primary winding and reset the magnetizing current to zero. In this process, the switch S1 must endure the voltage of Vin+Vr, which is much higher than input voltage. The magnetic reset circuit may be a RCD circuit, an auxiliary winding, or an active clamped circuit, of which the reset voltages are different according to the different circuit parameters. For general applications, the rated voltage of the switch S1 should be twice of input voltage. Due to higher rated voltage requirement for main switch, single-ended forward DC-to-DC converter is not suitable for high input voltage applications.
FIG. 2 shows the configuration and key operation waveforms of dual switch forward DC-to-DC converter. Two switches are employed in primary side. When the two switches turn on simultaneously, the transformer primary winding is connected to the input voltage and the energy is delivered from source to load. When the two switches turn off simultaneously, the magnetizing current passes by the two clamping diodes, which is denoted as Da1 and Da2 in FIG. 2. The input voltage is applied to primary winding reversely and reset magnetizing current to zero. Since the drain-to-source voltage of the switches is clamped by Da1 and Da2 to the input voltage, the switches only endure one time of the input voltage. This type converter is suitable for higher input voltage system.
However, the magnetic reset mechanism of dual switch forward converter is not optimal for wide input application. Since the reset voltage is equal to the input voltage, the reset time is also equal to the turn-on time of the switches in order to keep the voltage-second balance for transformer. With the decrease of the input voltage, the turn-on time should increase to output enough power. However, since the reset time also increases, the maximum duty cycle is limited within 50% in low input voltage. With the increase of the input voltage, the duty cycle becomes small and deteriorates the performance of the converter. By this reason this type converter is not suitable for wide input voltage applications.
In full-bridge type converter, four switches are employed in primary side, which turn on and turn off alternately to delivery the energy from source to load and keep the magnetic balance for transformer naturally. FIG. 3 shows a typical phase shift full bridge DC-to-DC converter. The key operation waveforms are shown in the figure as well.
In order to achieve the Zero-Voltage-Switching conditions for switches, phase-shift control logic is applied to four switches. Referring to FIG. 3, switches in each leg are driven by two compensative signals with fixed 50% duty cycle. That is, S1 and S4 turn on and turn off alternately, and S2 and S3 turn on and turn off alternately. The phase-shift of two legs is changeable. The overlap of S1 and S2 turn-on time is the effective duty cycle time for the transformer. So the output can be regulated by controlling the phase-shift of two legs.
It is easy to obtain the zero-voltage-switching condition for switches by using phase-shift control, however it brings out the circulation current problem. As shown in FIG. 3, in the time interval of S1 and S3 common turn-on or S2 and S4 common turn-on, the primary current circulates in primary switches and primary winding, and does not deliver energy to load. In wide input application, the effective duty cycle became smaller; the circulation current will cause larger conduction loss.
If the asymmetrical control logic is applied in full bridge structure, it also can regulate the output and obtain zero-voltage-switching for primary switches. FIG. 4 shows the conventional asymmetrical full bridge DC-to-DC converter and its key operation waveforms.
In FIG. 4, S1 and S2 turn on and turn off synchronously, S3 and S4 turn on and turn off synchronously. S1, S2 turn on with a duty cycle of D and S3 and S4 turn on with a duty cycle of 1-D. The block capacitor Cb, which is connected in series with the primary winding, offers a DC bias to keep the voltage-second balance for the transformer. Since both in S1, S2 turn-on and S3, S4 turn-on, the energy delivery process is going on, there is no more circulation current problem. But the magnetizing current has a DC bias, which will cause the trouble in transformer design. Another problem of this type converter is the non-linear control characteristic of output voltage to switching duty cycle. Duty cycle and conversion efficiency varies sharply with the variations of input and output voltage.
For the above reasons, a need still exists in the art of designing and manufacturing DC-to-DC converter to provide an optimal topology suitable for application to wide and high input voltage to accomplish several design objects. These design objects may include a goal of providing a DC-to-DC converter that imposes minimized voltage stress on switches, maximizes the switch duty cycle to ensure high conversion efficiency for wide operation range, and achieves soft-switching conditions for main switches. These design objects require new and improved converter configurations as will be described below.
In the preferred embodiment of the present invention, an asymmetrical full bridge circuit is presented which is used in a DC-to-DC converter. The circuit comprises two main switches and two auxiliary switches, a compensative capacitor connected in series with the branch circuit of the auxiliary switches. One main switch and one auxiliary switch form one leg of the full bridge, and the other main switch and the other auxiliary switch form the other leg of the full bridge. The primary winding of the transformer is connected to the center points of two legs.
Two main switches turn on and turn off synchronously, and two auxiliary switches turn on and turn off synchronously also. When two main switches turn on, two auxiliary switches remains OFF and the energy is transferred from source to load, when two main switches turn off, two auxiliary switches turn on, and reset the transformer. The compensative capacitor offers a DC bias to keep the voltage-second balance for the transformer.
In the further embodiment of this invention, the asymmetrical full bridge circuit further includes an extra inductor that connects in series with the primary winding of the transformer. At the end of reset process, this inductor assists the main switches to obtain the zero-voltage-switching condition.
In the further embodiment of this invention, the asymmetrical full bridge circuit further includes a saturable inductor that connects in series with the secondary winding of the transformer. At the end of reset process, this saturable inductor assists the main switches to obtain the zero-voltage-switching condition.
This invention proposes a new topology for the design of DC-to-DC converter. The advantages of this topology are presented in following aspects. First, voltage stress of all the switches including main switches and auxiliary switches is equal to the input voltage or less than the input voltage. This topology can be used in high input applications. Second, the duty cycle of the main switches can be greater than 50% in low input, and in whole input range the duty cycle is maximized and keeps higher conversion efficiency in whole input range. So this topology is suitable for wide input applications. Third, the soft-switching condition is always satisfied for auxiliary switches, and can be obtain for main switches by properly circuit designing. So this topology is suitable for higher switching frequency applications.
In the other embodiment of this invention, an asymmetrical full bridge DC-to-DC converter includes an asymmetrical full bridge circuit with two main switches two auxiliary switches and a compensative capacitor, a transformer with a primary winding and a secondary winding, and a rectification circuit with two rectifiers and a output inductor. Wherein, the secondary has a tapping point. Each terminal of the secondary winding connects a rectifier, and each rectifier connects to the output inductor and further connects to one terminal of the load. The other terminal of the load connects to the tapping point of the secondary winding.
This topology not only has the advantages of preferred embodiment of the present invention, but also has the features that the output voltage ripple is minimized and a smaller output inductor is needed.